Oxodimethyldisilacyclohexadienehomopolymer

ABSTRACT

A new siloxane polymer containing disilacyclohexadiene rings in the polymerackbone and a commercially viable method of making it are disclosed. The new polymer is an elastomer, and it has unsurpassed thermal stability. Some of the thermal, mechanical, and chemical properties of the new polymer are described.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention pertains to a composition of matter and a method ofpreparing it. More precisely, this invention involves the chemistry oforgano-silicon compounds and a new compound of this class. Specifically,this invention reveals the compoundpoly-1,4-oxo-1,4-dimethyl-1,4-disilacyclohexadiene and a method ofpreparing it. This compound is useful as an adhesive, insulator, pottingagent, and composite material constituent in any application requiringan elastomer for high temperature environments. Polymers ofdisilacyclohexadienes possess greater thermal stability than any otherknown family of polymers.

2. Description of the Related Art

The silicone rubber industry is based on a chemical process called thedirect process, wherein methyl chloride is reacted with elementalsilicon to produce mixed methylchlorosilanes. The monosiliconmethylchlorosilanes are then used to produce conventional siliconerubbers. The direct process produces a byproduct known as direct processresidue which consists of methylchlorosilanes having multiple siliconatoms. These higher methylchlorosilanes can be processed chemically tomonosilanes to increase the yield of the direct process, or they can beused as the starting material to make other organosilicon compounds.

The fraction of direct process residue that boils in the range 150°-160°C. is a source of methylchlorodisilanes. A method for isolating1,2-dimethyltetrachlorodisilane from this fraction and then convertingit to 1,2-dimethyltetramethoxydisilane has been reported by Watanabe etal in the Journal of Organometallic Chemistry, 128 (1977) 173-175.Watanabe's technique involves two steps. The disilane fraction is firstchlorinated with dry hydrogen chloride in the presence of aluminumchloride to convert the unwanted trimethyltrichlorodisilane intosym-dimethyltetrachlorodisilane. The sym-dimethyltetrachlorodisilane isnext purified by distillation and then treated with methylorthoformateto replace the chlorine atoms with methoxy groups. The result is1,2-dimethyltetramethoxydisilane. The 1,2-dimethyltetramethoxydisilaneresulting from Watanabe's technique can be converted intosym-1,4-dimethyl-l,4-dimethoxy-l,4- disilacyclohexadiene by the methoddisclosed in Atwell's U.S. Pat. No. 3,465,018. Atwell's method consistsin part of reacting a substituted tetramethoxydisilane precursor with anacetylene at elevated temperature to produce a compound with thedisilacyclohexadiene ring structure.

Example 7 in Atwell's patent shows a polymer made by exposing adihydroxy substituted disilacyclohexadiene ring compound to acidcatalyzed condensation polymerization. This is a standard polymerizationtechnique wherein water is eliminated from two hydroxyl groups, allowingan oxygen atom to bridge together two monomer molecules. The processcontinues until many monomers are connected. The resulting polymer,1,4-dimethyl-2,3,5,6-tetraphenyl- 1,4-polydisilacyclohexadienol, is theonly known disilacyclohexadiene ring polymer in the prior art. Thispolymer is reported to have a melting point ranging from 10° C. to 320 °C. Such a range of melting points indicates that the polymer is not anelastomer and that it has a very broad molecular weight distribution.Moreover, complete melting at 320° C. indicates that the thermalproperties are substantially inferior to other well known polymers,including many commercial polysilanes and polysiloxanes.

Disilacyclohexadiene ring polymers must be commercially viable to betruly useful. The diphenylacetylene used in the synthesis of Atwell'sring polymer is expensive and relatively rare. The present inventionuses ordinary, inexpensive acetylene gas to synthesize thedisilacyclohexadiene ring structure. The result is a ring withoutsubstituent groups at the 2,3,5,6 positions. The chemistry of this"naked ring" is different than that of a ring with bulky phenyl groupsattached to it. For this reason the diol of the unsubstituted ring couldnot be isolated. Attempts to do so led to uncontrolled polymerizationand useless gooey masses. A new approach to condensation polymerizationwould be needed to allow unsubstituted disilacyclohexadiene topolymerize in a controlled way.

While the diol of unsubstituted disilacyclohexadiene ring proved to bevery difficult to isolate, isolating the dipotassium salt of the diolproved to be straightforward. Moreover, the dipotassium salt can be madewithout first making the diol. The dipotassium salt was found to behavelike a base in the presence of acids, forming the potassium-acid saltand ring diol. The ring diol thus formed immediately polymerized in acontrolled manner. The use of a potassium salt as a polymer precursor isan unconventional technique that in this case solved an otherwiseintractable problem. The process can be understood as acid-basecondensation polymerization, and the dipotassium salt of the diol ofdisilacyclohexadiene makes it possible.

This invention provides a new disilacyclohexadiene polymer and a methodof preparing it. The new polymer, poly-1,4-oxo-1,4-dimethyl-1,4-disilacyclohexadiene, is an elastomer that exhibits thermalstability superior to any previously known polymer. This new polymer isprepared from inexpensive, widely available materials, providing thepotential for commercial manufacture.

SUMMARY OF THE INVENTION

This invention provides a new polymer with outstanding thermal stabilityand an economical method of preparing it. Starting with a byproduct ofthe silicone rubber industry called direct process residue,sym-dimethyltetrachlorodisilane is isolated by chlorination with HCl andAlCl₃ followed by distillation. This compound is then converted to1,2-dimethyltetramethoxydisilane by reaction with methylorthoformate.1,2-dimethyltetramethoxydisilane is then reacted With acetylene gas atelevated temperature to produce sym-1,4-dimethyl-1,4-dimethoxy-1,4-disilacyclohexadiene. This compound is thenreacted with potassium hydroxide to create1,4-dimethyl-1,4-disilacyclohexadiene-1,4-di(potassium silanoate). Thedipotassium disilanol salt is dissolved in ethanol and then reacted withHCl. Extraction with water and organic solvent followed by evaporationof the organic phase gives the elastomer poly-1,4-oxo-1,4-dimethyl-l,4-disilacyclohexadiene, characterized by thestructure: ##STR1## This homopolymer showed only a 5% weight loss bythermogravimetric analysis to 1000° C.

DETAILED DESCRIPTION AND EMBODIMENTS

The preparation of poly-1,4-oxo-1,4-dimethyl-1,4- disilacyclohexadienebegins with the fraction of direct process residue that boils between150° and 152° C. This fraction is refluxed in the presence of aluminumchloride while dry hydrogen chloride is bubbled into the mixture. Afterthe reaction is complete, the liquid is decanted and distilled withacetone. The fraction boiling between 150° and 152° C. is collected forthe next step. This fraction consists of almost puresym-dimethyltetrachlorodisilane. Methylorthoformate is next added to thedimethyltetrachlorodisilane and allowed to react for several hours in astirred vessel at about 70° C. Vacuum distillation of the resultingmixture gives symdimethyltetramethoxydisilane in the fraction boiling at82° C. The chemistry explained in this and the preceding paragraph wasfirst disclosed by Watanabe et al in the Journal of OrganometallicChemistry, 128(1977) 173-175.

The sym-dimethyltetramethoxydisilane is next reacted with acetylene inthe method disclosed by Atwell in U.S. Pat. No. 3,465,018. Nitrogen gascarries acetylene gas into a glass tube heated to 400° C. Into this gasstream, sym-dimethyltetramethoxydisilane is added dropwise. The liquidreaction product is collected in a condenser equipped flask at thedischarge end of the glass tube. The liquid product is then purified byvacuum distillation at less than 1 torr absolute pressure and 50° C. Theresulting product issym-1,4-dimethyl-1,4-dimethoxy1,4-disilacyclohexadiene with someimpurities.

The impure sym-1,4-dimethyl-1,4-dimethoxy-1,4-disilacyclohexadiene isnext reacted with potassium hydroxide dissolved in a methanol/watersolution by slowly adding the silane to the KOH solution. The solutionis then filtered and vacuum dried to remove most of the solvent.Tetrahydrofuran is then added, causing a precipitate to form. Theprecipitate is next filtered and washed with tetrahydrofuran and ether,and then recrystallized from isopropanol. A final wash of the solidswith isopropanol and pentane gives pure 1,4-dimethyl-1,4-disilacyclohexadiene- 1,4-di (potassium silanoate ). The1,4-dimethyl-1,4-disilacyclohexadiene-,4-di(potassium silanoate) is nextdissolved in ethanol. Concentrated hydrochloric acid is added to thesolution, causing potassium chloride to precipitate out. Water andmethylene chloride are then added to the solution, dissolving thepotassium chloride and forming a two phase mixture. The organic phase isthen decanted from the mixture. Upon evaporation of the methylenechloride at ambient temperature, an elastomer results. This elastomer isthe new compound poly-1,4-oxo1,4-dimethyl- 1,4-disilacyclohexadiene.

The preferred embodiment of this invention may be further understood byreferring to the following examples. These examples are given toillustrate but not limit this invention.

EXAMPLE Step 1. Preparation of dimethyltetrachlorodisilane from directprocess residue

Direct process residue was distilled to obtain the fraction that boilsbetween 150° and 152° C. 754.0 grams of this fraction were refluxed inthe presence of 54.9 grams of aluminum chloride, while dry HCl gas wasbubbled into the mixture. After refluxing for 34 hours and 12 minutes,the mixture was cooled and the liquid decanted into another vessel where50 ml of reagent grade acetone were added. This mixture was distilled,and 652.5 grams of the fraction boiling in the range of 150° to 152° Cwere collected. Gas chromatography showed the fraction to be 97% pure.Boiling point, infrared spectroscopy, and proton NMR were used topositively identify this fraction as sym- 1,2-dimethyl- 1,1,2,2-tetrachlorodisilane.

Step 2. Preparation of dimethyltetramethoxydisilane from1,2-dimethyl1,1,2,2-tetrachlorodisilane

In a stirred vessel at 68° C., 572.8 grams of methylorthoformate wereadded slowly to 466.0 grams of 1,2-dimethyl-1,1,2,2-tetrachlorodisilane. The mixture remained at 68° C. for 14 hoursand 21 minutes. The mixture was then vacuum distilled at 28 torr, and246.0 grams of the fraction boiling at 84° C were collected. Gaschromatography showed the fraction to be 97% pure. Boiling point,infrared spectroscopy and proton NMR were used to positively identifythis fraction as 1,2-dimethyl-1,1,2,2- tetramethoxydisilane.

Step 3. Preparation of1,4-dimethyl-1,4-dimethoxy-1,4-disilacyclohexadiene from1,2-dimethyl-1,1,2,2-tetramethoxydisilane and acetylene:

A pyrex tube 22 mm in diameter and 19 inches in length was heated tobetween 400 and 425° C. Nitrogen gas flowing at less than 10 ml/min wasmixed with 43 ml/min of acetylene gas and admitted into one end of thetube. 1,2-dimethyl-1,1,2,2-tetramethoxydisilane was added dropwise at arate of 0.142 ml/min to the inlet gas stream. Liquid reaction productwas collected at the tube discharge in a condenser equipped receivingflask. Gas chromatography indicated that the liquid product had two mainconstituents. One was identified by boiling point and peak retentiontime as methyltrimethoxysilane. Boiling point, infrared spectroscopy andproton NMR were used to positively identify the other constituent as1,4-dimethyl- 1,4- dimethoxy- 1,4-disilacyclohexadiene.

Excess heat during the distillation of the product caused polymerizationin the still. Partial purification was achieved by vacuum distillationat less than one torr absolute pressure and at temperatures below 50° C.This procedure provided1,4-dimethyl-1,4-dimethoxy-1,4-disilacyclohexadiene of 67% purity asshown by gas chromatography.

Step 4. Preparation of1,4-dimethyl-1,4-disilacyclohexadiene-1,4-di(potassium silanoate ) from1,4-dimethyl- 1,4-dimethoxy- 1,4-disilacyclohexadiene

16 ml of crude 1,4-dimethyl-1,4-dimethoxy-1,4-disilacyclohexadiene fromstep 3 were added slowly to 83.2 ml of 3.835 M KOH in 90% methanol and10% water. The resulting mixture was filtered and the filtrate wasvacuum evaporated at 60° C. to remove most of the solvent. 150 ml oftetrahydrofuran were then added, resulting in a precipitate and a twophase mixture. The precipitate was removed by filtration and washed witha small amount of tetrahydrofuran and ether. The precipitate was nextdissolved in 100 ml of boiling isopropanol. Upon cooling, a crystallinemass separated from solution. The crystals were filtered and washed with60 ml of isopropanol and 50 ml of pentane, followed by vacuumevaporation to remove residual solvent. 5.76 grams of crystals resulted.Proton NMR, infrared spectroscopy, and wet chemical techniques were usedto identify the crystals as being pure 1,4-dimethyl-l,4-disilacyclohexadiene-1,4di(potassium silanoate). Infrared absorptionbands were found at: 2890/cm (w), 810/cm (sb) for Si--CH₃ and 1340/cm(ms), 930/cm (sb), speculated to be the silane ring band. The proton NMRwas run in deuterium oxide and perdeuteroacetone, showing peaks at 0.0ppm for Si--CH₃ and 6.95 ppm for --CH=. The crystals were white, opaque,and very fine, and were found to be soluble in water, methanol, andethanol.

Step 5. Preparation of the homopolymerpoly-1,4-oxo-1,4-dimethyl-1,4disilacyclohexadiene from 1,4-dimethyl-1,4-disilacyclohexadiene- 1,4- di (potassium silanoate)

3.89 grams of 1,4-dimethyl- 1,4-disilacyclohexadiene- 1,4-di(potassiumsilanoate) from step 4 were dissolved in 10 ml of water in a stirredbeaker at ambient temperature and pressure. This solution was thentitrated to the phenolphthalein end point with glacial acetic acid,resulting in a precipitate of potassium acetate. The solution was nextextracted with ether, forming two phases. The organic phase was decantedand the ether was removed by vacuum evaporation. 1.59 grams of a tan,viscous polymer resulted. The polymer waspoly-1,4-oxo1,4-dimethyl-1,4-disilacyclohexadiene. Spectral propertieswere found to be: Infrared absorption bands at 2933/cm (mw), 1255/cm(m),822/cm (m), Si--CH₃ ; 1035/cm (s), Si--O--Si; and 1340/cm (m),associated with the ring. The proton NMR showed a singlet at 6.86 ppmfor -CH- and a very close doublet at 0.18 and 0.20 ppm for Si--CH₃.

EXAMPLE 2 Step 1. Preparation of1,4-dimethyl-1,4-disilacyclohexadiene-1,4-di(potassium silanoate)

The procedures of steps 1 through 4 of Example 1 were repeated toprovide starting material for this example. While the methods andreaction conditions were identical to those in example 1, the amounts ofmaterials used were increased proportionally to give a larger amount ofdipotassium salt to work with.

Step 2. Preparation of the homopolymerpoly-1,4-oxo-l,4-dimethyl-l,4disilacyclohexadiene from1,4-dimethyl-l,4-disilacyclohexadiene-1,4- di(potassium silanoate)

32.2 grams of 1,4-dimethyl-1,4-disilacyclohexadiene-1,4-di(potassiumsilanoate) were dissolved in 300 ml of ethanol in a stirred beaker atambient temperature and pressure, Concentrated hydrochloric acid wasthen added dropwise to the solution until reaching the phenolphthaleinend point after 20.85 ml of acid were consumed. A precipitate ofpotassium chloride resulted. 150 ml of methylene chloride and 220 ml ofwater were next added to the solution, dissolving the potassium chlorideand giving a two phase mixture. The organic phase was then decanted andpoured into teflon coated aluminum foil dishes. Upon evaporation of thesolvent at ambient temperature and pressure, an elastomer remained. Thiselastomer was poly-1,4-oxo-1,4-dimethyl-1,4-disilacyclohexadiene. Theelastomer lost only 5% of its weight in thermogravimetric analysis to1000° C. Some mechanical properties of the elastomer at ambienttemperature were found to be: Stress at breaking, 19.7 psi; Strain atbreaking, 26.2%; Work to break, 2.42 in-Lb/in³.

Obviously numerous modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:
 1. The elastomer compound poly-1, 4-oxo-1,4-dimethyl-1, 4-disilacylcohexadiene having infrared absorption bands at2933/cm(mw), 1255/cm(m), 822/cm(m), Si--CH₃, and 1035/cm(s), Si--O--Si,and 1340/cm(m), associated with the ring, and by proton NMR showing asinglet at 6.86 ppm for --CH--and a very close doublet at 0.18 ppm and0.20 ppm for Si--CH₃.
 2. A homopolymer elastomer characterized by therepeating group having the structure: ##STR2## and having infraredabsorption bands at 2933/cm(m.w), 1255/cm(m), 822/cm(m), Si--CH₃, and1035/cm(s), Si--O--Si, and 1340/cm(m), associated with the ring, and byproton NMR showing a singlet at 6.86 ppm for --CH-- and a very closedoublet at 0.18 ppm and 0.20 ppm for Si--CH₃.